Roller cone drill bits for drilling earth formations conventionally have roller cones mounted on bearing pins. As the bit rotates, the roller cones rotate on their respective bearing pins. Teeth formed on the roller cones, or inserts disposed in recesses on the roller cones, impact and crush the underlying earth formation material to form a borehole. Conventionally, bearings are disposed between the roller cones and the bearing pins to bear the forces to which the roller cones are subjected while the bit is rotated under applied axial force, commonly termed weight on bit, while allowing the roller cones to rotate on the bearing pins. The high forces present during drilling cause friction in the rotating components and generate heat, which may cause deterioration of the bearing. Bearing deterioration may cause bit failure, resulting in time-consuming and expensive removal and replacement of the bit from the borehole.
Friction or journal bearings used in roller cone bits may be sealed bearings or open bearings. Sealed bearing systems conventionally include a lubricant reservoir for supplying lubricant, such as a bearing grease, to the bearing surfaces between the roller cones and the bearing pins. A pressure compensator may be used to equalize the lubricant pressure with the fluid pressure within the borehole. Open bearing systems, by contrast, have no seals or bearing grease. Open bearing systems may use drilling fluid, such as a drilling mud, to both cool and lubricate the bearings.